1. Field of the Invention:
The present invention relates to a semiconductor laser device usable for an optical disk and the like.
2. Description of the Related Art:
In conventional semiconductor laser devices, a buried heterostructure such as that shown in FIG. 8 has been utilized for improving current/light confinement efficiently in a lateral direction (or a direction parallel to the surface of the substrate). A conventional semiconductor laser device of this type (hereinafter, such a device will be referred to as a "conventional semiconductor laser device (I)") will be briefly described with reference to FIG. 8. In this conventional semiconductor laser device (I), an n-type Al.sub.0.5 Ga.sub.0.5 As first cladding layer 502 (carrier density: 3.times.10.sup.18 cm.sup.-3 ; dopant: Se); an undoped active layer 503; a p-type Al.sub.0.5 Ga.sub.0.5 As second cladding layer 504 (carrier density: 5.times.10.sup.17 cm.sup.-3 ; dopant: Zn); and an n-type GaAs current/light confinement layer 505 (carrier density: 3.times.10.sup.18 cm.sup.-3 ; dopant: Se) are sequentially grown on an n-type GaAs semiconductor substrate 501 by performing a first metalorganic chemical vapor deposition (MOCVD) process. A stripe-shaped groove 506 is formed in the n-type current/light confinement layer 505 so as to reach the p-type second cladding layer 504. Then, by performing a second MOCVD process, a p-type Al.sub.0.5 Ga.sub.0.5 As third cladding layer 507 (carrier density: 1.times.10.sup.18 cm.sup.-3 ; dopant: Zn) and a p-type GaAs contact layer 508 (carrier density: 3.times.10.sup.18 cm.sup.-3 ; dopant: Zn) are formed thereon so as to bury the groove 506. In such a conventional semiconductor laser device (I), Se is generally used as an n-type dopant and Zn is generally used as a p-type dopant. In addition, in order to effectively confine current and light, the film thickness of the p-type second cladding layer 504 is set in an approximate range from 0.05 to 0.6 .mu.m.
On the other hand, Fujii et al. reported another type of semiconductor laser device in ICMOVPE VII conference (1994). Fujii's semiconductor laser device will be referred to as a conventional semiconductor laser device (II). In the conventional semiconductor laser device (II), first, an n-type Al.sub.0.5 Ga.sub.0.5 As layer (carrier density: 1.times.10.sup.18 cm.sup.-3 ; dopant: Se); a p-type Al.sub.0.5 Ga.sub.0.5 As layer (carrier density: 1.times.10.sup.18 cm.sup.-3 ; dopant: Zn; film thickness: 1.25 .mu.m) and an n-type GaAs layer are sequentially grown on an n-type GaAs substrate by performing an MOCVD process. In the case of using Se as a dopant for the n-type GaAs layer, when the carrier density of Se becomes 4.times.10.sup.18 cm.sup.-3 or more, a larger amount of Zn diffuses from the p-type Al.sub.0.5 Ga.sub.0.5 As layer to the n-type Al.sub.0.5 Ga.sub.0.5 As layer and the n-type GaAs layer. However, in the case of using Si as a dopant for the n-type GaAs layer, even when the carrier density of Si reaches 4.times.10.sup.18 cm.sup.-3, the diffusion amount of Zn is relatively small, and when the carrier density of Si becomes 6.times.10.sup.18 cm.sup.-3 or more, the diffusion amount of Zn is increased in the same way as in the case of Se.
Furthermore, Neave et al. reported still another type of semiconductor laser device in Appl. Phys. A32 (1983) 195. Neave's semiconductor laser device will be referred to as a conventional semiconductor laser device (III). In the conventional semiconductor laser device (III), in the case of using Si as an n-type dopant, when the carrier density of Si becomes 3.times.10.sup.18 cm.sup.-3 or more, the light-emission efficiency is abruptly decreased.
However, the conventional semiconductor laser devices (I) to (III) described above have the following problems.
In the conventional semiconductor laser device (I), an n-type first cladding layer 502; an active layer 503; a p-type second cladding layer 504; and an n-type current/light confinement layer 505 are sequentially grown on an n-type GaAs substrate 501 by performing a first MOCVD process. In this case, Zn contained as a dopant in the p-type second cladding layer 504 diffuses to both of the n-type first cladding layer 502 on the lower side and the n-type current/light confinement layer 505 on the upper side which sandwich the p-type second cladding layer 504 therebetween, so that the carrier density of Zn in the p-type second cladding layer 504 is decreased. As a result, the barrier of the p-type second cladding layer 504 with respect to the carriers injected into the active layer 503 becomes lower, so that the carriers leak into the p-type second cladding layer 504. Such a carrier leakage becomes remarkable when the semiconductor laser device operates at a high temperature. Thus, the characteristics of such a semiconductor laser device operating at a high temperature are adversely deteriorated.
On the other hand, in the conventional semiconductor laser device (II), in the case of sequentially depositing an n-type AlGaAs layer, a p-type AlGaAs layer and an n-type GaAs layer on an n-type GaAs substrate by performing an MOCVD process, if Si is used as a dopant for the n-type GaAs layer, then Zn or a dopant for the p-type AlGaAs layer diffuses less than the case of using Se as a dopant for the n-type GaAs layer as disclosed by Fujii et al. However, this multi-layer structure is different from a common semiconductor laser device structure in that a layer corresponding to an active layer for a common semiconductor laser device is not provided between the n-type AlGaAs layer and the p-type AlGaAs layer in the conventional semiconductor laser device (II). Moreover, the film thickness of the p-type AlGaAs layer is 1.25 .mu.m, which is far larger than a typical film thickness (in an approximate range from 0.05 .mu.m to 0.6 .mu.m) of a second cladding layer in a semiconductor laser device structure. Furthermore, the carrier density of the p-type AlGaAs layer is unclear. Therefore, it is not clear how Zn diffuses when this structure is applied to a semiconductor laser device structure.
Furthermore, in the conventional semiconductor laser device (III), when high-density Si is doped as an n-type dopant, the light-emission efficiency is degraded as reported by Neave et al. Therefore, in the case of using Si as a dopant for the n-type cladding layer adjacent to the active layer for improving the characteristics of a semiconductor laser device operating at a high temperature, Si diffuses into the active layer, so that Si disadvantageously degrades the light emission efficiency inside the active layer.